Swiss needle cast (SNC), a foliage disease of Douglas-fir (Pseudotsuga menziesii), is caused by the physical blockage of the stomata by pseudothecial ascocarps of the fungus Phaeocryptopus gaeumannii. Occlusion of the stomata results in a decrease in the ability of the host to exchange gases with the environment leading to reduced carbon assimilation (Manter et al., 2000). This results in premature foliage loss and subsequent growth reduction due to decreased photosynthetic leaf area (Manter et al., 2003). Prior to the 1980’s, it was known to cause severe disease only in exotic Douglas-fir plantings and was considered an innocuous needle endophyte throughout the native range of Douglas-fir, where it historically caused little impact (Hansen et al., 2000). It has since emerged as a significant forest health issue in the western Coast Ranges of Oregon and Washington for reasons that are not well understood. Climate has a significant influence on the abundance of P. gaeumannii, with factors such as mean-daily winter temperature and spring/summer precipitation being the best predictors of disease severity (Stone et al., 2008). Changes in the local climate on the Oregon coast, with winter temperatures increasing significantly in recent decades, may be a contributing factor to the intensification of SNC (Stone et al., 2008). There may also be variation in the virulence of two coexisting fungal lineages that may have some influence in the recent emergence (Winton, 2001). This study aims to determine the distributions of two previously identified cryptic lineages of P. gaeumannii in the Pacific Northwest, and assess spatial variation in genotypic diversity and population structure using DNA microsatellites known as simple sequence repeats (SSRs). This information will be important in determining whether the distribution of these lineages, or the genetic structure of their populations, is associated with SNC disease severity at the landscape level.
Bennett, Patrick; Stone, Jeff. 2017. Hierarchical population structure of Phaeocryptopus gaeumannii in the Pacific Northwest: From needles to landscapes. In: Goodrich, B.; Palacios, P., comps. Proceedings of the 64th Annual Western International Forest Disease Work Conference, 2016 May 9-13, Sitka, AK. p. 19-25.